Sorry for the wall of text, it’s one of those subjects I’m invested into.

I have two bags of criticism against it: pragmatic and moral.

Pragmatic

The whole thing breaks for countries associated with multiple languages. What’s the flag for India supposed to represent: Hindi, Bengali, Tamil, one of the local English varieties? (Yes. Plus hundreds more.) If someone speaks Tamil, a Hindi version of the site (or game, or whatever) is as useful as a Finnish version for an English speaker - but you’re still misguiding their attention to a version they won’t use.

It also breaks for languages associated with multiple countries. Should a Spanish speaker in Argentina look for the flag of Spain, Argentina, Mexico, USA (Yahoo does/did this), or what? And, if they do find the flag of Spain, should they keep looking for a random LatAm country flag? (They’d probably prefer a LatAm Spanish version of the site, but in its absence Peninsular Spanish is fine.)

Those aren’t even exceptions, they’re examples of the RULE (Done! I’m in 196, sharing rule content! :-P ). The territory controlled by a country will most likely have at least some linguistic minority, and you’re likely to find speakers of a language in at least two of those territories.

And the problem only gets worse and worse as you add support to more languages. Multiple copies of the same flag for different languages quickly become noise; and you’re likely to hit more and more languages where “it’s the language of $country” is not obvious, so you’re sending your user to a visual quest to find the “right” flag.

Now, how many language versions are there in your site? If it’s only a handful, it’s trivial to scan the language names; and if it’s a lot, the problem gets worse because now you’re seeing multiple languages being associated with the same flag, and multiple misdirections. Either way, it’s simply not worth the effort.

Ah, let us not forget that ~10% of the world pop has a hard time with colours, and flags are coloured symbols.

So you’re introducing a lot of unnecessary complexity, for both the webdesigner and the user; neither needs it. Just list languages by their native names (as you should already be doing); and if the amount of languages is large enough to justify it, let users search it by name (as you should already be doing). Done.

Moral

Minority speakers already face at the very least disdain, if not hostility from the general population. But specially from the governments they pay taxes to; Welsh Not is a mild example of that that, but if you want to see things going rogue look at the former Ottoman empire. Or just, you know, the Nakba? It isn’t just a religious matter, but also linguistic…

At the very least it’s that consistent and pestering pressure to conform to something you are not. To ditch part of your identity because you’re “an irregular”, and you should be “fixed”. Always surrounded by all those government symbols - specially the flag.

We certainly don’t need sites and games also telling you “hey, this symbol is associated with «your» country, so this is supposed to be your language. Be clean, speak it”.

But even for majority speakers, the whole “you’re getting another flag for your language” is bound to bleed their insecurities. Represent Portuguese by the flag of Portugal, and you’re telling people in Brazil “you don’t even speak «proper» Portuguese”; do it by the flag of Brazil and you’re telling people in Portugal “your Portuguese varieties are some irrelevant whatever”. (Which end of the stick Portuguese speakers in Angola and Mozambique get? Yes.)

And some flags are intrinsically problematic. Flags represent governments; and governments are always fucking it up, as those things like to pretend they’re above actual human beings. Russian speakers in Ukraine should be able to use their preferred language, without having it associated with a government literally killing them… who am I joking, they’re right in the area most affected by the war. At least Nazi Germany kicked the bucket, otherwise you’d be seeing its flags everywhere.

And the whole mindset that this encourages is… nasty. It’s always that “one country, one language” discourse rooted in nationalism. Why are we still wallowing in a 1700 meme that provides one of the pillars for fascism?

As a Canadian, I never know if I’m supposed to be looking for the Stars and Stripes or Union Jack.